A mobile telecommunication network, like a UMTS network comprises cells in which a primary station (or NodeB) communicates with a plurality of secondary stations (or User Equipments or UEs). The cells are controlled by Network Controllers.
In UMTS Long Term Evolution (LTE), eMBMS services (evolved MBMS) can be provided by a single frequency network (SFN) in which groups of cells are coordinated and synchronised to perform a simulcast MBSFN transmission from multiple cells, which is seen as a single transmission by a secondary station. MBSFN traffic can be either sent:                on a dedicated frequency layer, or        on a frequency layer shared with non-eMBMS services, where sets of “mixed cells” support both unicast and MBSFN transmissions.        
As in UTRA Release 6 MBMS deployment, in LTE it will still be possible for single cells (SC) to send point-to-multipoint (PTM) eMBMS traffic: in LTE this will be transmitted on the DL-SCH (Downlink shared channel) channel and scheduled by the primary station for each individual cell.
It has been decided that this Point To Multipoint from Single Cell (PTM-SC) configuration will support a dedicated feedback channel (identical to the Unicast case) that will allow HARQ for acknowledging/Non-acknowledging (ACK/NACK) the eMBMS transmissions and for the eNodeB to schedule retransmissions on DL-SCH.
In MBSFN networks, eMBMS traffic will be transmitted point-to-multipoint from multi-cells (PTM-MC) on the MCH channel: this traffic will be scheduled by an MCE control entity (Multicast Coordination Entity). It has not yet been decided if this PTM-MC configuration will support a mechanism for feedback, whether on a dedicated or a common channel. Similarly, the possibility of providing multi-cell retransmissions (across the whole or part of the MBSFN network) is still an issue open.
As noted above, discussions are ongoing in 3GPP RAN2 on whether the HARQ mechanism proposed to support retransmissions for PTM-SC configurations may be extended to PTM-MC configurations: the benefit in terms of network efficiency is recognized, but the complexity of implementation makes the adoption of a suitable mechanism unclear.
In particular, MBSFN retransmissions may not be sent on the MCH channel, as MCH transmissions are scheduled on the MCCH: any MBSFN-wide retransmissions may be more appropriate for repetitions of services rather than isolated retransmissions. Isolated MBSFN retransmissions can on the other hand be sent on the DL-SCH in a mixed cell, where they can be scheduled by the mixed cell eNodeB.
In addition, coordination between eNodeBs will be needed in MBSFN networks in order to optimize coverage and throughput (modulation and coding scheme) in PTM-MC configurations. It will be advantageous for the network to have specific information on e.g. the MBSFN area edge users since they will experience poorer performance in the MBSFN area: since radio conditions between unicast operation and MBSFN operation can differ, feedback information from individual users on cell coverage or channel quality (e.g. physical layer measurement or CQI reporting) may be required specifically in order to assist cell planning and resource provisioning.
No technical proposal has yet been presented for the provision of individual or aggregated (common) feedback on the cell coverage and channel quality in PTM-MC configurations in MBSFN networks.
Furthermore, the feedback resources allocation for unicast UEs having not yet been decided, the eMBMS feedback resources cannot yet be specified either (for both PTM-SC and PTM-MC configurations), since for resource efficiency it is preferred to re-use the same mechanism for unicast and eMBMS traffic: the main concern with providing feedback is the additional overhead incurred in the uplink, and if a specific mechanism is agreed for unicast then it is likely to be selected too for eMBMS PTM-SC and/or PTM-MC, albeit with some adaptations for the multi-cells configurations.
The exact implementation details of the dedicated uplink feedback channel for unicast (and PTM-SC) have not yet been fixed, however the current line of agreement (in 3GPP TS 36.300) for unicast feedback is that some form of RACH resource partitioning will take place where a number of RACH resources (signature preambles known as random IDs) are allocated for random selection by UEs wishing to transmit feedback.
Generally however, there is no mechanism available yet for any type of common feedback (physical measurement, CQI, ACK/NACK for HARQ, etc) from multiple UEs in point-to-multipoint configurations in MBSFN networks for more than a few UEs: it has been proposed that point-to-point unicast feedback may be used in lieu of point-to-multipoint feedback for up to 10 UEs, but there is a consensus that for reasons of performance and interference, this scheme will not be scalable to a higher number of UEs in either PTM-SC or PTM-MC operation.